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December 16th, 2014

No Advantage for Low-Glycemic-Index Diet

In recent years the glycemic index (GI), a measure of a carbohydrate’s impact on blood sugar, has assumed a major role in discussions about diets and nutrition. Now a study suggests that by itself, within the context of an otherwise healthy diet, GI may not be an important factor in improving cardiovascular risk.

In a paper published in JAMA, Frank Sacks and colleagues report the results of a randomized, crossover-controlled, 5-week feeding trial comparing four different diets in 163 overweight or obese adults. The diets were either low- or high-carbohydrate and either low- or high-GI. Importantly, all the diets were based on previously established healthy dietary patterns based on the DASH diet, which is low in saturated and total fat and includes substantial amounts of fruits, vegetables, and low-fat dairy foods.

The findings did not suggest any major benefits associated with low GI. In the main comparison of interest, the low-GI, low-carbohydrate diet — when compared with the high-GI, high-carbohydrate diet — lowered triglycerides from 111 to 86 mg/dL but had no significant impact on insulin sensitivity, systolic blood pressure, LDL cholesterol, or HDL cholesterol. Unexpectedly, in a comparison of the low and high-GI diets within the high-carbohydrate group, insulin sensitivity was decreased and LDL was increased in the low-GI group.

The authors caution that their trial did not attempt to “address the effect of glycemic index in a typical US diet.” They also warn that they did not study the effect of lowering GI in people who have type 2 diabetes or for weight loss. Current evidence suggests that a low GI diet may be helpful in these situations.

Another factor to consider is that because “nutrients often cluster… the effects of glycemic index, if any, might actually result from other nutrients, such as fiber, potassium, and polyphenols, which favorably affect health.”

In an accompanying editorial, Robert Eckel writes that the “unexpected findings of the study… suggest that the concept of glycemic index is less important than previously thought, especially in the context of an overall healthy diet…. These findings should therefore direct attention back to the importance of maintaining an overall heart-healthy lifestyle, including diet pattern.”